Angiogenesis is the process whereby new blood vessels sprout from existing vessels and requires that the specialized resident cells lining the vasculature, the endothelial cells (ECs), proliferate, migrate and differentiate spatially and temporally in response to specific signals. Vasculogenesis, on the other hand, has only recently emerged as an alternative mechanism of blood vessel growth in adult tissues and is the result of homing and engraftment of circulating EC precursors (ECPs) of bone marrow origin to areas of neovascularization. Both events are known to occur within the liver vasculature under very different conditions of growth, injury and repair, but the extent of each and the mechanisms by which they proceed in each case is completely unknown. The overall goal of this proposal is to determine the specific growth factor signaling events that induce angiogenic versus vasculogenic blood vessel growth in the context of liver and determine the subsequent microenvironmental milieu that induces EPC recruitment and/or differentiation of the liver-specific sinusoidal endothelial cell (SEC) fenestrated morphotype. Two paradigms of clinically-relevant liver repair allow us to spatially and temporally detail the growth factor signaling events and evaluate sinusoidal ultrastructure and liver-specific SEC function in the rat. AIM I: Growth factor and microenvironmental interactions will be examined at the sinusoidal surface following 70% partial hepatectomy (PHx) since there is initially extensive hepatocyte proliferation in the absence of EC proliferation until 96 hr post-PHx, resulting in avascular hepatic foci. Subsequent proliferation and infiltration of the SEC into these avascular parenchymal clusters and reestablishment of the normal hepatic architecture provides a well-timed model for evaluating physiological angiogenesis. AIM II: Angiogenic and vasculogenic events will be examined concurrently using an allogeneic liver transplant model (dual cross strain and female to male transplants, non-GFP to GFP rats) to ascertain the source of SEC (host vs. graft) and determine extent of recipient and donor involvement in SEC engraftment and/or proliferation. Prior to transplantation, livers are stored under cold, ischemic conditions for 18 h, and upon transplantation (warm reperfusion), a majority of the SEC slough off immediately from the sinusoidal surface. Remarkably, within 24 hr, the SEC lining has nearly completely repopulated at least partially from ECPs, but microvascular remodeling, morphological and functional modification occurs over subsequent days. Comparative analysis of these two systems will elucidate both similar and dissimilar growth factor signaling mechanisms and the role of the microenvironment that control these events and potentially lead to optimization of therapies that will reflect the specific requirements for injury based liver neovascularization.